Looks like it was, though, unless you mean the "jointly developed by Microsoft and IBM" bit.
Edit: to clarify, when people say "DOS", 99.999% of the time they mean MS-DOS, not the generic "disk operating system", which is a category and wasn't developed by anyone in particular.
Edit: to clarify, when people say "DOS", 99.999% of the time they mean MS-DOS, not the generic "disk operating system", which is a category and wasn't developed by anyone in particular.